Power rheology model to assess bacteria growth in activated sludge polluted with phenanthrene

Abstract

Environmental biotechnology is a scientific and engineering system that use microorganisms to detoxificate solid, liquid and gaseous wastes. Activated sludge is a biotechnology that clean up sewage and industrial wastewaters using air and biological flocs. Different mathematical models expressed the bacteria growth in relation to the sludge parameters. The Power model described the relationship between the shear rates of the sludges with hydrocarbon and <i>Leptothrix</i> spp. flocs sizes; and was expressed as: Ƭ = K . ɣⁿ. Where: (Ƭ) was the shear stress, (K) indicated the sludge viscosity, (ɣ) was the shear rates and η was the flow behavior index. The assays were performed to evaluate the thick wooly flocs formation with and without phenanthrene. At low inoculum sizes (10⁵ cells/m³), most of the flocs were larger than 3 µm, and the cellular yield was 0.3-0.6 mg dry wt/ml. In contrast, with higher inoculums (10⁹ cells/m³), flocs of 2 µm were observed, and the bacterial yield was around a constant value c.a. 1.6 g dry wt/m. The relation of the inoculum with the flocs sizes responded to an hyperbolic curve. This phenomenon was related to poor growth due to oxygen limitation and hydrocarbon presence inside the large flocs. The experimental data were tested by the Power model, and the R² obtained indicated the goodness of the fit to the bioassays. The optimization of bioremediation strategies like the bacterial bioaugmentation in activated sludge, confirmed that the Power model provided the best prediction of viscosity that determined the aeration with the suspended solids and phenanthrene present in the sludges.